By no one's count do advocates of the Senate package — and, particularly, of the compromise from Manchin and Toomey — have the votes to beat a filibuster. Compiling data from news accounts, a ProPublica survey of legislators, and the Huffington Post's similar overview, we estimate that there are 53 likely supporters of the compromise to 41 opponents. This will almost certainly change over the course of the day; a senator-by-senator breakdown is at the bottom of this post, including sources.

Interestingly, the Daily News is reporting that Republican leadership in the Senate isn't hustling its members to vote against the bill.

Republicans senators should “vote their conscience,” Senate Minority Whip John Cornyn (R-Texas) the second ranking GOP senator, said Monday in explaining that he does not plan to “whip” against the vote to amend a broader gun control measure to add the background check deal.

In order to entice additional support, the Huffington Post reports that Manchin and Toomey are pitching an alteration to the bill. In its original iteration, it would cover any private sale, including those at gun shows but exempting sales between members of the same family.

The language has not been finalized. But talks have centered on exempting gun sellers whose businesses are located far from a federal firearms licensee -- perhaps outside a 100-mile radius -- from performing background checks on gun buyers.

The revised plan would serve as a lure for rural lawmakers who have worried that forcing commercial sales to go through FFLs would be overly burdensome for those in far-off places. Manchin and Toomey are hopeful that one of those lawmakers, Sen. Lisa Murkowski (R-Alaska), will put forward the change.

For those keeping track, this would be an amendment to the Manchin-Toomey amendment, which amends Sen. Chuck Schumer's existing proposal in the package, a universal implementation of background checks that has only limited support.

The week promises to be a tumultuous one, with an unclear outcome. The National Journal explains the various votes and debates that can be expected before Senate action is finalized, and quotes a House Republican leader suggesting that bipartisanship would go a long way on the other side of the Capitol.

“If it’s an 80-plus vote in the Senate, it’s going to be hard to slow down over here,” a House GOP leadership aide said. On the other hand, if the bill barely makes it out of the upper chamber, “that’s when we can really take a look at what the Senate sent us with a magnifying glass,” the aide said.